The development of fiber-reinforced polymeric composite materials has been of great benefit as these materials can provide excellent strength characteristics and corrosion resistance at low densities. Polymeric composites have been of particular benefit in the transportation industry, where the ability to produce light weight, strong, and resistant panels from polymeric components has greatly enhanced efficiency of vehicles and decreased both construction and operating costs.
Unfortunately, utilization of fiber-reinforced polymeric composites to their full potential, and in particular as replacement for traditional metal-based materials, remains limited due to high costs, a large portion of which can be attributed to assembly. Costs associated with assembly and adequate fastening of polymeric-based composites can be significant as connecting bonds lack the presence of the reinforcement materials and generally rely on the resin matrix alone for bond strength. To insure formation of the strongest possible welds, the bond must provide the fullest possible contact and matrix participation throughout the entire bond area.
To improve polymeric composite assembly methods and outcomes, joining methods such as induction welding have been developed. Induction welding is a non-contact welding process that encourages fusion bonding between components. A process uses alternating magnetic fields to induce eddy currents and thereby heat magnetically susceptible materials (susceptors) within and/or adjacent to the bond area. A typical induction welding device includes an alternating current generator, a coil to generate the AC magnetic fields, and often a ferromagnetic element that can channel and direct the magnetic fields to the susceptor located within/near the bond. A fusion bond is created when thermoplastic polymers at the joining surfaces are heated by the susceptors to the melting or softening point as the surfaces are held in contact with one another. The surfaces are generally held in contact under pressure while the materials cool below the softening temperatures to complete the bonding process. When considering bonding between two thermoplastic composites, the matrix materials at the bonding surfaces can mix and form a fusion bond. When considering thermoset-based composites, a thermoplastic material associated with the susceptor and/or at the surface of the thermosets can function as an adhesive to bond the composites.
A variety of forms and materials have been examined for use as susceptors. Typically, induction welding methods include the placement of a welding interlayer susceptor tape within the bond area. The welding tape generally includes the susceptor and thermoplastic resin either coating or sandwiching the susceptor. Conventional susceptors are metallic and have a regular pattern of openings. Unfortunately, achieving a uniform, controllable temperature in the bond area, which is crucial to preparing a weld of adequate integrity to permit use of induction welding in high stress applications such as in the transportation industry, is difficult with these conventional susceptors.
Additives of polymeric composites have also been utilized as susceptors. While this method is possible, e.g., composite additives such as graphite or boron fibers can be heated directly by induction, it presents certain challenges. The exponential decay of the strength of magnetic fields dictates that, in induction welding processes, the susceptible structure closest to the induction coil will be the hottest, since it experiences the strongest field. As such, it is difficult to obtain adequate heating in the bond area without overheating areas of the composite away from the bond and closer to the coil. The matrix resin in a large area or even in the entire composite can overheat and melt, which can result in porosity in the product, delamination, and, in some cases, destruction or denaturing of the polymer. Composites that include magnetically susceptible additives can successfully be welded by induction, but they require use of a susceptor of significantly higher conductivity than the additive to selectively peak the heating in the bond area.
What are needed in the art are susceptors that can be individually designed for and applied to fiber reinforced polymeric composites according to the particular composite components, geometry, and use. Improved design and application of susceptors can provide improved control and uniformity of temperature in the bond area and can provide welds with improved strength and integrity.